A conventional amperometric determination of chemical components (hereinafter referred to as "analytes") in a solution is performed by immersing a number of electrodes, typically three, in the analyte solution. During electrochemical measurements an electrolyte, such as an ion forming salt or any other ion forming compound which when dissolved increases the conductivity of the solution, is usually added to the analyte solution. The three electrodes are referred to as working electrode (WE), reference electrode (RE) and counter electrode (CE). At least one working electrode is required, but there is no restriction to the use of any number of additional working electrodes, if this is advantageous for a specific desired type of measurement.
The reference electrode is designed to maintain a constant and well-defined electrical potential. With respect to this potential a fixed electrical potential, negative or positive, is applied on the working electrode by means of an external electronic device, a potentiostat. If analytes in the electrolyte solution being exposed to this potential undergoes reaction at the working electrode, an electrical current will flow between the working electrode and the counter electrode. This current may be measured, and reflects the concentration of the analyte(s) in the solution, while the direction of the measured current flow depends on whether an oxidation or a reduction process is present at the working electrode.
Ideally, no current flows through the reference electrode of a triple electrode system as described above. In the simplest case, the current measured between the working electrode and the counter electrode is directly proportional to the analyte concentration. The chemical reactions occurring at the counter electrode are usually of no interest from a measuring point of view. Therefore, the counter electrode is often separated from the analyte solution and is placed in a separate receptacle in a liquid being electrically connected with the analyte solution via a porous sintered glass filter.
In a similar way, instead of a potentiometer a control equipment delivering a constant current can be used, whereby a resulting potential is measured across the working electrode.
Under certain conditions the reference electrode and the counter electrode may be combined. This is possible when the processes occurring at the counter electrode are reversible with regard to a component in the electrolyte, so that the rate of the electrochemical processes at the combined electrode is not current limiting for measuring an analyte at the working electrode.
A difficulty experienced during amperometric measurements is to achieve well-defined conditions in the solution at the boundary layer adjacent to the working electrode. Such well-defined conditions are of importance for obtaining reliable measurement results from which valid conclusions, such as determination of concentration, may be drawn. The rate at which processes occur at the working electrodes is regulated by the rate of electron transfer across the boundary layer between the analyte in the solution and the electrode, and/or by the rate of analyte transport through the solution to this boundary layer.
In order to achieve a well-defined transport of analyte towards the electrode the working electrode is rotated. Therefore, this is usually formed as a cylindrical body provided with one or several electrodes, usually working electrodes. In a typical embodiment, a central disk-shaped disk electrode and outside this an annular ring electrode, are positioned on the flat end surface whereby both these electrodes are working electrodes.
Such a configuration is called a rotating ring-disk electrode. In addition to working electrodes, reference and counter electrodes could be integrated in the rotatable body as well.
The U.S. Pat. No. 4,889,608 to G. Eickmann describes an electrode system including one electrode, wherein the electrode is disposed in a replaceable electrode fixture, which electrode fixture is mounted via an adapter on an elongated solid metal shaft, the shaft also acting as an electric conductor. When changing an electrode, the electrode fixture is dismounted to allow replacement of the electrode.
Replacement of an electrode according to U.S. Pat. No. 4,889,608 requires a special tool. Alternatively, the entire electrode fixture is replaced. However, the electrode fixture includes several components making it comparatively expensive. Furthermore, the electrode system is adapted for the case of a single electrode only.
Therefore, there is a need for a simplified replaceable electrode system for an arbitrary number of electrodes.